1. Field of the Invention
This invention relates to a photographic lens of a large aperture ratio and of a so-called partial focus type which is arranged to perform focusing by moving a part of an optical system while keeping the total length of the optical system (length from a first lens surface to an image surface) unvaried.
2. Description of the Prior Art
Among tele-photographic lenses, the lenses of the rear focus type arranged to perform focusing by moving at least one of the lens groups disposed on the image side of the front part thereof have been known from U.S. Pat. Nos. 4,045,128 and 4,068,929. In these photographic lenses, the heavy front part of the lens system remains stationary during a focusing operation. Therefore, the center of gravity of the lens system moves to a smaller extent during focusing. Besides, since it is a light weight lens group that is to be moved, a focusing operation can be performed easily with a light touch.
When a lens group is moved for focusing, the movement of the lens group brings about changes of some aberrations. However, when the focal length of the whole lens system is arranged to be short, an aberration that is not conspicuous under a long focal length condition will increase to degrade the quality of the picture taken. Hence, a tele-photographic lens capable of correcting such degrading aberrations has been proposed in U.S. Pat. No. 4,240,703.
However, it has been a general tendency for the conventional rear focus type photographic lenses that the lens has a relatively small aperture ratio and thus has a large F number, because: a tele-photographic lens of a large aperture ratio tends to have a long total optical length and, accordingly poor operability.